An earthquake, measured at 5.1 on the Richter scale, marked the start of the 1980 eruption of Mount Saint Helens. The earthquake triggered a massive collapse of the north face of the mountain, which slid downslope, entraining ice, rock and explosive materials, and giving rise to a massive pyroclastic flow (a volcanically-induced avalanche) which flattened vegetation and buildings in an area of over 150 square miles (approximately 389 km²).

For more than nine hours, a vigorous plume of ash erupted, eventually reaching 12 to 15 miles (20-25 km) above sea level. The plume moved eastward at an average speed of 60 miles per hour (95 kilometers/hour), with ash reaching Idaho by noon. By early May 19, the devastating eruption was over.

The pyroclastic flow was one of the largest witnessed in modern memory. Moments after its formation, the flow crossed Spirit Lake and passed right over a ridge 1,300 feet (395 m) high. The flow then continued for 14 miles (22 km) down the Toutle River[?].

Noticeable ash fell in eleven states. The total volume of ash (before its compaction by rainfall) was approximately 0.26 cubic mile (1.01 km³), or enough ash to cover a football field to a depth of 150 miles (240 km).

An 84-year old innkeeper who had lived near the mountain for over 50 years became nationally famous when he declined to evacuate before the impending eruption, despite repeated pleas by local authorities. The body of Harry R. Truman was never found. In total, fifty-seven people were killed or never found.